1. Field of the Invention
The present invention relates to a portable, internal combustion engined power tool, in particular a setting tool for driving in fastening elements, and including a combustion chamber, at least one suction/discharge valve connected wit the combustion chamber, and an operational piston which performs an operational stroke upon ignition of a fuel gas mixture filling the combustion chamber. The present invention also relates to a method of controlling operation of such a power tool.
2. Description of the Prior Act
With a power tool of a type described above, the drive energy is generated by burning a fuel gas mixture in a combustion chamber and is transmitted by the operational piston to a fastening element. Upon pressing a power tool against a constructional component into which a fastening element is to be driven, an ignitable fuel gas mixture is injected into the combustion chamber. Upon actuation of a trigger, a spark is produced which ignites the fuel gas mixture, initiating a combustion process. The operational piston, which adjoins the combustion chamber, is driven by the combustion gases. At the end of its operational stroke, the piston passes past discharge openings through which the waste gases can be at least partially discharged. The piston, after having performed the operational strokes, returns in its initial position as a result of underpressure which was created in the combustion chamber by cooling of the waste or residual gases. During the period of thermal return of the piston to its initial position, the combustion chamber should remain sealed from the surrounding it environment. Therefore, the suction/discharge valves, which provide for delivery of fresh air into the combustion chamber, should be open only after the piston has returned into its initial position. Generally, the time necessary for the return of the piston into its initial position increases with the increase of the tool temperature which heats during operation. In addition, high-energy power tools require a large expansion volume which results in that a greater time becomes necessary for the return of the piston into its initial position.
In some conventional power tools, closing of a suction/discharge valve can be effected with a pawl connected by, e.g., a toggle lever with the trigger. In this way, the suction/discharge valve becomes open as soon as the trigger returns into its initial position. This means that by the time the trigger returns to its initial position, the piston also must return into its initial position.
The locking of the suction/discharge valve by the trigger means that the switching point of the trigger cannot any more be arbitrary selected. The ignition switch can only then be actuated when the locking of the suction/discharge valve has been completed, i.e., long after the start of the displacement of the trigger. However, a long trigger displacement adversely affects acceptance by the customers. Moreover, as it has already been discussed above, with a heated tool, the return of the piston into its initial position lasts longer. The tool user must, in this case, hold the trigger in its pulled condition longer to prevent the piston from occupying a erroneous position.
In order to increase the time available for return of the piston into its initial position, the trigger displacement can be damped. However, damping negatively influences operational characteristics of the trigger as a larger force is needed for actuating the trigger, and the trigger does not return sufficiently rapidly into its initial position. A user does not look at dampening favorably as it reduces the maximum setting rate and requires a greater force for actuating the trigger, which the user has to apply.
German Publication DE 19962 598.0 suggests detecting of the gas pressure in the combustion chamber after the fuel gas mixture has been ignited and locking the suction discharge valve(s) dependent on the detected gas pressure.
Accordingly, an object of the present invention is to provide a method which would permit to precisely determine when the piston returns into its initial position and thereby would provide for a more precise control of release of the suction/discharge valve.
Another object of the present invention is to provide a power tool which would contain means that would permit more precisely determine the return of the piston into its initial position.
These and other objects of the present invention, which would become apparent hereafter, are achieved by providing a method according to which the displacement position off the piston is inquired, and the suction/discharge valve is released after the return of the piston in its initial position has been ascertained.
The tool according to the present invention includes inquiry means for inquiring the piston position and which actuates a locking/release device associated with the suction/discharge valve for opening same.
According to the present invention, the method for operating a portable internal combustion engined power tool including a combustion chamber, at least one suction/discharge valve for supplying fresh air into the combustion chamber and for discharging waste gases therefrom, and an operational piston displaceable in an operational direction upon ignition of a fuel gas mixture filling the combustion chamber, includes inquiring a displacement position of the piston when it is being displaced to its initial position after having performed an operational stroke; and releasing the at least one valve upon the piston reaching its initial position.
The foregoing method permits to precisely determined the geometrical position of the piston, in particular, its initial position. The release or opening of the suction/discharge valve is effected only when the piston has reached its initial position. The pressure variations of the residual gases in the combustion chamber do not lead any more to a faulty control during the valve release. The danger of the valve being released before th piston reaches its initial position is completely eliminated. The initial position of the piston is a position in which the opening between the combustion chamber and the guide cylinder is completely closed.
In principle, for inquiring the displacement position of the piston, a position of any portion of the piston can be monitored. However, because the piston is guided in a guide cylinder and has a piston rod at least a portion of which projects from the guide cylinder, according to embodiments of the invention, it is the position of the piston rod which is being ascertained. In this case, the access to the inside of the cylinder or the combustion chamber is not any more necessary. Therefore, the inquiring element or device for determining the displacement position of the piston can be formed much simpler, and it can be mounted much easier. The displacement position of the piston can be easily ascertained from the displacement position of the piston rod. Based on the principle that it is the initial position of the piston that need be ascertained, a corresponding clearly defined point on the piston rod can be monitored, e.g., by using a stationary sensor. Passing of the defined point of the piston rod past the sensor, upon returning of the piston to its initial position, is a clear indication that the piston has reached its initial position.
According to one embodiment of the present invention, as a defined point which has to be monitored for determining the displacement position of the piston rod, the end surface of the piston rod is used. The position of the end surface of the piston rod can be monitored very easily and very precisely. This permits to precisely determine the return of the piston to its initial position.
In case, the piston rod has not returned to its initial position after the performance of the operational stroke, according to a further development of the present invention, a forceful release of the suction/discharge valve is provided for. The forceful release is provided in order to discharge the waste or residual gases from the combustion chamber. In this case, the piston must be brought into its initial position by other means, namely, manually or by a subsequent ignition of a new fuel gas mixture injectable into the combustion chamber.
According to the present invention, the portable, internal combustion engined power tool includes a combustion chamber, at least one suction/discharge valve for supplying fresh air into the combustion chamber, and for discharging waste gases therefrom, an operational piston displaceable in an operational direction upon ignition of a fuel gas mixture filling the combustion chamber, a locking/release device for release the at least one suction/discharge valve after the piston has reached its initial position after having performed operational stroke, and inquiry means for inquiring a displacement position of the piston when it is being displaced to its initial position after having performed operational stroke and for actuating the locking/release device when the piston has reached its initial position.
According to the invention, it is, thus, provided for a piston location-dependent control of the locking/release device, with the control being independent from pressure variation of the residual gases in the region of the combustion chamber.
As it has already been discussed previously, the piston, which is displaceable in a guide cylinder, has a piston rod a portion of which projects from the guide cylinder. Therefore, according to the invention, the inquiry element for determining the position of the piston is located outside of the guide cylinder in vicinity of the guide cylinder. This simplifies the design of the tool and insures an easy mounting and monitoring of the inquiry element. This position of the inquiry element insures a precise determination of the displacement position of the piston rod.
The piston rod and the inquiry element are located immediately opposite each other, whereby the inquiry of the displacement position of the piston rod is effected directly.
As it has already been discussed above, for inquiring the displacement position of the piston, a displacement position of the piston rod is monitored. As it has further been discussed above, a clearly defined point on the piston rod, which corresponds to the initial position of the piston, can be monitored. As it has still further been discussed above, as a defined point on the piston rod, the piston rod end surface is used, which is monitored with suitable sensor means. As sensor means mechanical, electrical, optical, and magnetic sensors can be used. Besides the end surface of the piston rod, as a defined point, other geometrical elements of the piston or the piston rod can be used. Also, an external elements mounted on the piston or the piston rod, such as magnets, soft iron cores, optical bar codes, can be used. However, the most advantageous element for ascertaining the return of the piston into its initial position, proved to be the end surface of the piston rod as its position can be most easily detected.
According to one embodiment of the present invention, an inquiring element can be formed as a sensor located adjacent to the displacement path of the piston rod and generating an electrical release signal as soon as the piston rod leaves the region of the sensor. The sensor generates the electrical release signal when the free end of the piston rod passes the sensor, which position of the piston rod corresponds to the initial position of the piston. To this end, the distance of the location of the sensor from the initial position of the piston is so selected that the sensor is located immediately in front of the end surface of the piston rod when the piston occupies its initial position.
The electrical release signal actuates the locking/release device which releases the suction/discharge valve(s). When the locking/release device has a locking lever, the electrical release signal can be used for a actuating an setting device that lifts the locking lever off its locking position. The locking lever is brought into its locking position by the tool trigger. The setting device retains the locking lever in its locking position until it is actuated by the electrical release signal.
According to another embodiment of the present invention, the inquiry element for inquiring or ascertaining the displacement position of the piston rod is formed as a resilient feeler biased against the circumference of the piston rod and extending into the displacement path of the piston rod when the piston has returned to its initial position. In the initial position of the piston, the feeler end is located immediately in front of the free end surface of the piston rod. Forming the inquiry element as a feeler simplifies its structure and, at the same time, provides for an inquiry element which is robust and require little maintenance. It is also not sensitive to contamination. The end of the feeler adjacent to the piston rod can be formed as a spring element, in form, e.g., of a leaf spring with a convex end, with the piston rod extending tangentially to the convex end in the displacement direction of the piston.
The locking/release device locks or releases the suction/discharge valve(s) dependent on whether the feeler engages the piston rod or extends into the piston rod displacement path, respectively. The locking or release of the valve(s) can be effected by the locking lever of the locking/release device engageable by the feeler.
The feeler can be connected with the free end of the locking lever which is displaceable into its locking position by the tool trigger. The locking lever remains in its locking position after the ignition of the fuel gas mixture in the combustion chamber as long as the feeler engages the circumference of the piston rod.
According to an advantageous embodiment of the present invention, there is provided a setting device that displaces the locking lever in its release position as soon as the piston has returned into its initial position. As the feeler element is formed as a mechanical element engageable with the circumference of the position rod, it is formed as a resilient element as it need be compressed upon the pivotal movement of the locking lever in its release position.
As a setting device, e.g., an actuation button, which is actuated manually and is connected with the locking lever, can be used.
However, the setting device can be also formed as an angular lever pivotally supported at its apex, with one leg of the angular displacing the locking lever of the locking/release device into its locking position when the other leg of the angular lever is displaced by a press-on element, which is supported in the tool housing, upon the displacement of the press-on element to the rear of the tool. The displacement of the other leg provides for pivoting of the angular lever in a respective direction. If, for some reasons, the piston has not returned in its initial position after performing its operational stroke, the tool can again be pressed against the constructional component to displace the locking/release device or its locking lever into the release position, which insures release of the suction/discharge valves(s) and deaeration of the combustion chamber. With a subsequent press-against process, the fuel gas again is injected into the combustion chamber, and the working process is conducted with the piston spaced from its initial position. However, in this condition of the piston, no fastening element is located in the outlet channel of the tool, so that there is no any danger of injury. Upon return of the piston in its initial position, the normal operation of the tool is resumed.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to is construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.